1. FIELD OF THE INVENTION
The present invention relates to a novel cooling apparatus for efficiently conducting and radiating heat from semiconductor chips or high-density integrated circuits, and to a semiconductor device combined with the cooling apparatus, as well as a mounting structure for mounting such a semiconductor device on a computer.
2. DESCRIPTION OF THE RELATED ART
Modern large-scale computers are required to cope with high speed processings, and the scale of integration of individual integrated circuits has been increasing. As a result, the quantity of heat generated by each integrated circuit has increased substantially. In this situation, great concern has been raised about the excessive rises in the temperature of integrated circuits which have become common and upon which no special emphasis has previously been placed. In particular, the cooling of integrated circuits has become an important problem to be overcome in the current mounting techniques used for mounting integrated circuit chips on large-scale computers. Also, in order to minimize the length of electrical wires for connecting integrated circuit chips, a method of incorporating a multiplicity of integrated circuit chips into a micropackage has been developed.
In the field of large-scale computer systems in particular, a cooling apparatus of the type as shown in FIG. 4 has previously been proposed. The cooling apparatus illustrated in FIG. 4 is based on a gas-filled thermal conduction cooling system of the type disclosed in Japanese Patent Unexamined Publication No. 61-15353. Large scale integrated circuit (hereinafter referred to as "LSI(s)") chips 11 (only one of which is shown) are connected through extremely small solder balls 17 to a multi-layer printed-wiring board (hereinafter referred to as "board") 8 constituted by a multiplicity of electrically conductive layers and electrically insulating layers, and, in turn, are electrically connected to a multiplicity of pins 12 which project from the reverse side of the multi-layer board 8. A housing 15 is mounted on the board 8 to cover the LSI chips 11. A plurality of cylinders 18 are provided within the housing 15, and each of the cylinders 18 includes a piston 13 for conducting heat from a back surface of the corresponding LSI chip 11 and a spring 14 for applying pressure to the piston 13. The space defined between the board 8 and the housing 15 is charged with a helium gas having good heat conductivity.
Heat generated from the LSI chip 11 is conducted to the piston 13 through heat resistance Rc in the LSI chip 11 and heat resistance Rc p at the contact portion between a spherical end of the piston 13 and the back surface of the LSI chip 11. The heat is further conducted to the housing 15 through the heat resistance Rf of the piston 13 itself and the heat resistance Rp.sub.-- h of the helium charged in the space between the piston 13 and the cylinder 18. This heat in turn is radiated through the heat resistance Rh of the housing 15 and the heat resistance Rexf between the housing 15 and the cold water which circulates through an upper portion of the housing 15 or a cooler 16 for circulating a coolant in the latter stage of the cooling process.
Japanese Patent Unexamined Publication No. 58-91665 discloses a structure which is constituted by a similar combination of a piston and a cylinder. In this structure, a piston element is pressed radially as well and is brought into contact with the cylinder.
However, the heat conducting device having the prior art structures described above are limited in capacity, and it is difficult to cool chips having a rating of several tens or more of watts per chip.
In general, the heat conductivity of a helium gas is excellent, about ten times that of other gases, but is extremely low as compared with the heat conductivity of such metal members as the piston 13 and the cylinder 18. Accordingly, the heat resistance Rp.sub.-- h of the helium gas (layer) exhibits the highest heat resistance. In order to lower the heat resistance Rp.sub.- h, it is necessary to reduce the space between the piston 13 and the cylinder 18. For this reason, the piston 13 or the cylinder 18 must be worked with high precision. Low precision working would downgrade the state of movement of the piston 13, and this may lead to fluctuations in the temperatures of the LSI chips 11.
Japanese Patent Unexamined Publication No. 61-15353 discloses an example in which a medium having good conductivity is charged into the spaces between the pistons 13 and the cylinders 18 in place of a helium gas. In this example, a satisfactory effect cannot be achieved without using a medium of this sort having good conductivity. Furthermore, the piston 13 is formed into a conical shape so as to increase the area of its heat conducting surface, and a suitable level of contact pressure is applied to the grease between the piston 13 and the cylinder 18. However, no direct contact can be obtained between the cylinder 18 and the piston 13, and the heat conductivity of typical types of grease is extremely low, about one-tenth of that which subsists in the case of direct contact.
The above-described prior art involves various other problems. None of the pistons is arranged to follow variations in the position of a corresponding semiconductor device. A complicated structure is needed and, if no heat conducting medium is used, it is impossible to lower the heat resistance. Accordingly, none of the aforesaid cooling apparatus are able to satisfy the requirements of current large-scale computer systems.